A. Field of Invention
The present invention relates to phosphorescent paints and coatings and coated articles.
B. Background
Phosphorescent pigments have long been incorporated into paints and plastics as a means of imparting phosphorescent properties to formed products and coated substrates. Though use of phosphorescent pigments has often been relegated to novelty, glow-in-the-dark items, at least one important use of phosphorescing products, namely phosphorescing paints and adhesive tapes, has been for demarcating building exits, egress pathways and potential obstacles to egress, such as stairs, to facilitate building evacuation. Phosphorescent paints and adhesive tapes are particularly desirable for use in these applications because, by them, building exits and egress pathways can remain visible to a departing occupant even in blackout conditions.
Recent large-scale building evacuations following natural and man-made disasters have prompted new consideration of the use of phosphorescent materials for the demarcation of building exits and egress pathways and of the performance characteristics of those materials. This has prompted a need to develop phosphorescent pigments having greater brightness and increased afterglow and to form products with them and methods for using those products that are adapted to maximize the performance characteristics of the enhanced phosphorescent pigments.
There are, at present, essentially two classes of phosphorescent pigment chemistries for use in forming phosphorescent products. Sulfide phosphorescent pigments, which includes compounds such as CaS:Bi, CaStS:Bi, ZnS:Cu, ZnCdS:Cu and the like, have been available for decades. Sulfide phosphorescent pigments, however, are chemically unstable, have a relatively short afterglow, and may contain cadmium. These pigments have been successfully used in forming water based phospholuminescent coatings. Notwithstanding their compatibility with and stability in water-based coatings, these pigments do not have the brightness and afterglow necessary to form water-based coatings that can meet modern performance expectations.
Recently, a new class of phosphorescent pigments has been made commercially available. This class of phosphorescent pigments includes the metal aluminates, particularly alkaline earth aluminate oxides, of the formula MAl2O4 where M is a metal or mixture of metals. Examples of these metal aluminate pigments include strontium aluminum oxide (SrAl2O4), calcium aluminum oxide (CaAl2O4), barium aluminum oxide (BaAl2O4) and mixtures thereof. The metal aluminate pigments show significantly improved brightness and afterglow in comparison to the sulfide phosphorescent pigments. Thus, they are desirable to use in many applications.
Many alkaline earth aluminate oxides, however, will hydrolyze in the presence of water to form salts. This presents a challenge in using these metal aluminate oxides in water-based coatings. In latex coating compositions, for example, the salts will rapidly destabilize the latex emulsion and increase the pH. The increasing pH leads to further hydrolysis and further destabilization of the emulsion. Destabilization of the latex emulsion is ultimately evidenced by the gelling or irreversible separation of the coating composition and effectively hampers the usefulness of the composition as a coating.
Various approaches have been undertaken to modify metal aluminate pigment particles to make them more stable for use in water-based coatings. Approaches to stabilizing metal aluminate pigment particles have, for example, involved coating the particles with or encapsulating them in a moisture barrier. For example, U.S. Pat. No. 6,242,043 describes firing metal aluminate phosphors in the presence of ammonium fluoride or ammonium diflouride at relatively high temperatures to impart onto the phosphor a moisture impervious coating. Potassium borosilicate glass coated metal aluminate pigment particles are also available and show improved stability in water. U.S. Pat. Nos. 5,665,793 and 5,874,491, each to Anders, teach compositions containing a moisture sensitive luminescent substance, including metal aluminate oxide phosphors, and an epoxy resin or water miscible polyurethane resin. To these compositions, it is taught that an amount of water-based paint may be blended. Without describing a specific mechanism, these patents suggest that the polyurethane or epoxy protects the pigment from being attacked by the moisture in the air and the water in the water based paint composition. No shelf life improvement is noted however.
Despite some indications that treated metal aluminate pigments are more stable in water based coatings, treated metal aluminate pigments are more expensive to use than untreated pigments and can suffer comparatively reduced brightness and afterglow.
It still remains to create a cost-effective, stable, phosphorescent, water-based coating composition that enjoys the benefits of using metal aluminate pigments. Water-based coatings are desirable for their ease of application, durability, and in light of environmental considerations, as water based coatings may be formed having relatively low solvent levels and VOCs. It is particularly desirable to develop water-based coating that incorporate metal aluminate pigments and yet retain a substantial, useful shelf life, during which time the phosphorescence will not substantially degrade. The present invention describes a novel solution for making a phosphorescent, water-based coating that incorporates untreated metal aluminate pigments.